separations Article Hydrophilic Interaction Liquid Chromatography to Characterize Nutraceuticals and Food Supplements Based on Flavanols and Related Compounds Oscar Vidal-Casanella 1 , Kevin Arias-Alpizar 1 , Oscar Nuñez 1,2,3 and Javier Saurina 1,2,* 1 Department of Chemical Engineering and Analytical Chemistry, University of Barcelona. Martí i Franquès, 1-11, E-08028 Barcelona, Spain; [email protected] (O.V.-C.); [email protected] (K.A.-A.); [email protected] (O.N.) 2 Research Institute in Food Nutrition and Food Safety, University of Barcelona, Recinte Torribera, Av. Prat de la Riba 171, Edifici de Recerca (Gaudí), Santa Coloma de Gramenet, E-08921 Barcelona, Spain 3 Serra Húnter Fellow, Generalitat de Catalunya, E-08008 Barcelona, Spain * Correspondence: [email protected] Abstract: Purified extracts from different types of berries and medicinal plants are increasingly used as raw materials for the production of nutraceuticals and dietary supplements, mainly due to their high content in bioactive substances. This is, for instance, the case of phenolic compounds such as flavonoids, which exhibit a wide range of beneficial properties, including antioxidant, anti- inflammatory, antineoplastic and antimicrobial activities. This paper tackles the characterization of several kinds of nutraceuticals based on hydrophilic interaction liquid chromatography (HILIC) with fluorescence detection (FLD). The study focused on the determination of flavanols and re- lated compounds such as condensed tannins. Analytes were recovered by solvent extraction using methanol:water:hydrochloric acid (70:29:1 v:v:v) as the extraction solvent under sonication for 30 min Citation: Vidal-Casanella, O.; at 55 ◦C. Experimental design with response surface methodology was used to optimize the HILIC Arias-Alpizar, K.; Nuñez, O.; Saurina, separation to achieve good resolution of the main components, using acetonitrile:acetic acid (99:1 v/v) J. Hydrophilic Interaction Liquid Chromatography to Characterize (solvent A) and methanol:water:acetic acid, (95:3:2 v/v/v) (solvent B) as the components of the mobile Nutraceuticals and Food phase. For the assessment of the elution gradient, factors under study were solvent B percentage Supplements Based on Flavanols and and gradient time. The best conditions were achieved with 10% solvent B as the initial percentage Related Compounds. Separations 2021, and 30 min of linear gradient to reach 25% solvent B. Principal component analysis and partial least 8, 17. https://doi.org/10.3390/ square-discriminant analysis were used to characterize and compare the compositional features of separations8020017 dietary supplements based on both targeted and non-targeted approaches. Results revealed that the sample distribution relied on the oligomeric nature of descriptors. Academic Editor: Jared L. Anderson Received: 14 January 2021 Keywords: hydrophilic interaction liquid chromatography; fluorescence detection; nutraceuticals; Accepted: 3 February 2021 flavanols; fingerprinting; principal component analysis Published: 9 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. The consumption of fruits and vegetables has been related to a decrease in the risk of suffering chronic diseases and, in parallel, to an increment of longevity [1]. Several studies have proved that some phytochemicals from fruits and vegetables can play a bioac- tive role, thus helping to prevent some diseases and bringing positive effects for human health [2]. Among other products, the berries such as cranberry, blackberry, blueberry and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. raspberry have gained the attention of consumers because of their exceptional polypheno- This article is an open access article lic content that provides great antioxidant, anti-inflammatory, antiviral and antibacterial distributed under the terms and activities [3–6]. In the case of artichoke-based products, widely used in the formulation conditions of the Creative Commons of dietary supplements, their hypolipidemic activity has been partly attributed to some Attribution (CC BY) license (https:// bioactive polyphenolic agents [7,8]. Furthermore, other vegetal products from grapevines creativecommons.org/licenses/by/ and medicinal herbs are increasingly used as ingredients in nutraceuticals because of their 4.0/). multiple beneficial detox and medicinal properties. Separations 2021, 8, 17. https://doi.org/10.3390/separations8020017 https://www.mdpi.com/journal/separations Separations 2021, 8, 17 2 of 12 Phenolic compounds consist of a vast group of phytochemicals, with more than 8000 different molecules, which are the most important source of antioxidants in the human diet. They are often classified according to their chemical structures, being phenolic acids and flavonoids the principal families. Focusing on flavonoids, the basic skeleton consists of two phenyl rings linked by three carbon atoms that form an oxygen heterocycle ring. This family is divided into six groups differing in the degree of hydrogenation and hydroxylation of the three-ring system, namely: flavonols, flavones, isoflavones, flavanones, anthocyanidins and flavanols [9]. The latter includes highly relevant compounds such as catechin, epicatechin and their oligomers, the so-called proanthocyanidins (PAC) or condensed tannins. PACs are formed from the linkage of flavanol monomers, being the single bond between C4 of the chain-extension unit and C6 or C8 of the chain-terminating moiety; the most common one (B-type link). Some molecules have an additional interflavan link, the so-called A-type, which consist of an oxygen bridge between C2 and C7 of the flavanol monomers (Figure S1). It should be noted that although A-type compounds are much less common and abundant, they are powerful natural antimicrobial compounds to be used in the prevention and treatment of some infections [10]. In more detail, A-type compounds prevent the adhesion of bacteria to tissue walls, so they can be used in the prophylaxis of urinary and respiratory infections, among others. [11]. The determination of phenolic compounds, in particular flavanol-related species, is typically carried out by liquid chromatography (LC). Reversed-phase (RP) mode with silica-based C8 or C18 columns is the most reasonable choice for the determination of phenolic acids, flavonoids, stilbenoids and other phenolic compounds in food matrices. RP separations offer an excellent performance for dealing with small molecules, both aglycon and glycosylated species, based on their different polarities. However, this approach has been found to be limited for the separation of larger molecules such as tannins including PAC oligomers. As a successful alternative, Hammerstone et al. demonstrated the poten- tiality of normal-phase (NP) high performane liquid chromatography (HPLC) with UV and fluorescence detection to determine PACs in cocoa and chocolate samples [12]. The method was able to separate compounds from monomers to decamers, while bigger polymers re- sulted in broader bands. However, the lack of available commercial standards for oligomers (degree of polymerization (DP) > 3) hinders the identification and quantification of these species. The application of liquid chromatography coupled to MS detection techniques (LC-MS) offered new possibilities for identifying the ion fragments of the specific PACs. Mass spectral data can also be used to characterize structural features of the PACs, and distinguish between singly and doubly linked proanthocyanidins (A-type or B-type) [13]. More recently, the introduction of hydrophilic interaction liquid chromatography (HILIC) has offered new analytical possibilities for the separation of bioactive molecules [14]. In this regard, various authors have explored the possibilities of HILIC separation, in general based on LC-MS(/MS) platforms, for the analysis of condensed tannins in various food matrices, such as in the case of cocoa [15], blackcurrant and other berries of Finnish com- mercial cultivars [16,17], apple extracts from different varieties [18,19] and dry-blanched peanut skins [20]. This paper deals with the development and application of HILIC for the characteri- zation of various nutraceutical and functional food products based on flavanol profiling and fingerprinting. The HILIC mode has been established using a bare silica column as the stationary phase and acidified acetonitrile and methanol solutions as the components of the mobile phase. The optimization of the separation relies on experimental design for a systematic exploration of the influence of the solvent composition and the elution gradient profiles. Owing to the complexity in the composition of some sample extracts, the best separation corresponds to that leading to the maximum number of resolved peaks in a minimum run time. In this regard, the separation performance has been investigated using a cranberry extract as a representative sample. The method has been applied to the analysis of commercial food supplements and the resulting data from both profiling and fingerprinting approaches has been exploited for sample characterization. In this Separations 2021, 8, 17 3 of 12 regard, principal component analysis (PCA) has been used for more efficient data explo- ration. Results revealed important differences in sample patterns which clearly depend on compositional issues. 2. Materials and Methods 2.1. Chemicals and Solutions General reagents and solvents